When considering the small intestinal phase of meal assimilation, the bile constituents we are most concerned with are the bile acids. These form structures known as micelles that serve to shield the hydrophobic products of lipid digestion from the aqueous environment of the lumen. Bile acids are in essence biological detergents, and large quantities are needed on a daily basis for optimal lipid absorption—as much as 1 to 2 g/day. The majority of the bile acid pool is recycled from the intestine back to the liver after each meal via the enterohepatic circulation (Fig.
30.7 ). Thus bile acids are synthesized in a conjugated form that limits their ability to passively cross the epithelium •Fig. 30.7 Enterohepaticcirculationofbileacids.Activeuptakeofconjugatedbileacidsoccursviatheapicalsodium-dependentbileacidtransporter(asbt).
lining the intestine so that they are retained in the lumen to participate in lipid assimilation (discussed in detail under that heading). However, when the meal contents reach the terminal ileum, after lipid absorption has been completed, the conjugated bile acids are reabsorbed by a symporter, the apical Na+-dependent bile acid transporter (asbt), that specifically takes up conjugated bile acids in association with sodium ions. Only a minor portion of the bile acid pool is left to spill over into the colon in health, and here bile acids become deconjugated and subject to passive reabsorption (see
Fig. 30.7).
The net effect is to cycle the majority of the bile acid pool between the liver and intestine on a daily basis, coincident with signals arising in the postprandial period. Bile acids also exert biological actions beyond their role as detergents by binding to both cell surface and nuclear receptors in a variety of cell types throughout the body. In this way they regulate their own synthesis as well as other metabolic processes.
The most important physiological function of the small intestine is to take up the products of digestion of ingested nutrients. Quantitatively the most significant nutrients (macronutrients) fall into three classes: carbohydrates, proteins, and lipids. The small intestine is critical not only
The Small Intestinal Phase of the Integrated Response to a Meal for absorption of nutrients into the body but also for the final stages of their digestion into molecules that are simple enough to be transported across the intestinal epithelium. We will consider the processes involved in assimilation of each of these nutrients in turn, beginning with carbohydrates. Carbohydrate digestion occurs in two phases: in the lumen of the intestine and then on the surface of enterocytes in a process known as brush border digestion. The latter is important in generating simple absorbable sugars only at the point where they can be absorbed. This may therefore limit their exposure to the small number of bacteria present in the small intestinal lumen that might otherwise use these sugars as nutrients.
Digestion of Carbohydrates
Dietary carbohydrates are composed of several different molecular classes. Starch, the first of these, is a mixture of both straightand branched-chain polymers of glucose. The straight-chain polymers are called amylose and the branched-chain molecules are called amylopectin (Fig.